Preparation of mixed vanadium phosphorus oxide catalysts and their use in oxidative processes

ABSTRACT

Vanadium phosphorus mixed oxide containing catalysts are prepared by sequentially forming an alpha-VOPO 4  catalyst precursor in aqueous media, and thereafter partially reducing a portion of the vanadium to a valence state of +4 in an organic liquid reducing media, in the absence of corrosive reducing agents. The catalysts exhibit excellent activity for the production of maleic anhydride from 4 carbon atom hydrocarbons such as n-butane.

BACKGROUND OF THE INVENTION

This invention relates to a method for preparing catalysts useful in theoxidation of hydrocarbons. More particularly it is directed to thepreparation of catalysts suitable for the production of dicarboxylicacid anhydrides from hydrocarbons, such as the production of maleicanhydride from 4-carbon atom hydrocarbons, such as n-butane, n-butenes,1,3-butadiene or a mixture thereof.

Catalysts containing vanadium and phosphorus oxides have been used inthe oxidation of 4-carbon atom hydrocarbons, such as n-butane,n-butenes, 1,3-butadiene or mixtures thereof with molecular oxygen oroxygen-containing gas to produce maleic anhydride. Conventional methodsof preparing these catalysts involve reducing a pentavalent vanadiumcompound, and combining the same with a phosphorus compound and, ifdesired, promoter element compounds under conditions which will provideor maintain vanadium in a valence state below +5 to form catalystprecursors capable of being converted to an oxide. The catalyst oxideprecursor is then recovered and calcined to provide active catalyticmaterial.

U.S. Pat. No. 3,985,775 describes the preparation of vanadium phosphoruscatalysts in an aqueous solution of concentrated hydrochloric acid. Thispatent also describes the catalyst preparation in non-aqueous media,where corrosive reducing agents were added to the media or weregenerated in situ.

The use of gaseous HCl as a reducing agent for vanadium is disclosed inU.S. Pat. No. 4,002,650 where the vanadium and phosphorus components arereacted in an aqueous solution. The use of gaseous HCl as a reducingagent for vanadium is also described in U.S. Pat. No. 4,043,943 wherethe vanadium and phosphorus components are reacted in a liquid organicmedium.

U.S. Pat. No. 4,016,105 describes the preparation of vanadium andphosphorus oxide-containing catalysts, utilizing as reducing agents,organic acids or aldehydes, together with a co-reducing secondaryalcohol. These reducing agents are added to an aqueous solution with thevanadium and phosphorus components.

Similar preparational techniques are described in European PatentApplication No. 3,431 in which the additional step of comminuting thevanadium-phosphorus precursor to a particle size of 500 to 700 microns(0.5 to 0.7 mm) is disclosed.

The use of such reducing agents as disclosed in the art requires specialprecautions in the preparation of these catalysts because of thecorrosive nature of the materials utilized. The process of the presentinvention permits the preparation of mixed vanadium phosphorus oxidecatalyst without the use of corrosive reducing agents.

A method for preparing catalysts containing vanadium and phosphorusoxides as described in U.S. Pat. No. 4,132,670, which required themaintenance of a solid phase and dispersion of the vanadium-containingfeed compound. The method includes forming a vanadium-containingcompound dispersion in an organic liquid medium such as alcohols,aldehydes, ketones, ethers or mixtures thereof, heating the dispersionto reduce the vanadium, and thereafter adding phosphoric acid in anorganic solvent.

The preparation of oxidation catalysts containing the mixed oxides ofvanadium and phosphorus is disclosed in U.S. Pat. No. 4,244,879 whereina vanadium compound is at least partially solubilized in an organicliquid medium capable of reducing at least a portion of the vanadium toa +4 valence state, and unsolubilized vanadium having a particle sizelarger than about 0.1 mm diameter is removed from the medium beforeaddition of a phosphorus-containing compound.

The preparation of vanadium phosphorus mixed oxide containing catalystsis disclosed in co-pending U.S. Ser. No. 146,971, now U.S. Pat. No.4,333,853 assigned to our common assignee, wherein partial reduction ofa pentavalent vanadium compound is effected in the presence of aphosphorus compound in an organic liquid medium capable of reducing thevanadium.

Co-pending U.S. Ser. No. 220,629, now U.S. Pat. No. 4,361,501 alsoassigned to our common assignee, discloses the preparation of vanadiumphosphorus oxide catalysts utilizing a mixed phosphorus componentcompound source, preparing the catalyst in a liquid medium capable ofreducing the vanadium component.

Generally, the preparation of vanadium phosphorus mixed oxide containingcatalysts in organic media rather than aqueous media, resulted in theproduction of catalysts having higher activity for oxidation reactions,such as the oxidation of n-butane to produce maleic anhydride. Theorganic-derived vanadium phosphorus mixed oxide catalysts additionallyexhibit higher intrinsic surface area than the aqueous-derived vanadiumphosphorus mixed oxide catalysts. Although the use of organicpreparative media capable of properly reducing the vanadium component ofthe catalyst alleviated the need to employ corrosive reducing agentswith their attendant costs and difficulties, the commercial use of atotal organic preparation would be less desirable than a method ofpreparation which limited the use of organic materials to criticalprocessing steps.

DISCLOSURE OF THE INVENTION

We have found that vanadium phosphorus mixed oxide containing catalystsexhibiting excellent hydrocarbon oxidation activity can be prepared by aprocess which includes the formation of a substantially pentavalentalpha-VOPO₄ catalyst precursor in aqueous media. The alpha-VOPO₄catalyst precursor is then subjected to vanadium reduction in asubstantially organic liquid medium capable of reducing at least aportion of the vanadium to a +4 valence state. This approach is contraryto teachings of the prior art, which teachings generally instruct thereduction of vanadium prior to formation of a vanadium-phosphorusprecursor.

The catalysts prepared by the method of the present invention exhibitexcellent activity for hydrocarbon oxidation reactions, such as theproduction of maleic anhydride from 4-carbon atom hydrocarbons,particularly n-butane.

It is therefore an object of the invention to provide a process forpreparing vanadium and phosphorus mixed oxide-containing catalystsuseful for the oxidation of 4-carbon atom hydrocarbons such as n-butaneto produce maleic anhydride, which catalysts exhibit excellent yieldsand selectivity to maleic anhydride.

It is a further object of the invention to provide a process forpreparing vanadium and phosphorus-containing catalysts useful for theoxidation of 4-carbon atom hydrocarbons such as n-butane, to producemaleic anhydride, which process is simplified, highly reproducible, andeconomical; which process avoids the hazards of corrosion, and which iscapable of commercial scale-up.

In general, the process of the present invention includes

(a) introducing a pentavalent vanadium compound and a pentavalentphosphorus compound into an aqueous medium;

(b) forming a pentavalent, alpha-VOPO₄ catalyst precursor in the aqueousmedium;

(c) recovering the pentavalent catalyst precursor from the aqueousmedium.

(d) introducing the pentavalent catalyst precursor into a substantiallyorganic liquid medium capable of reducing at least a portion of thevanadium to a valence state of about +4 in the absence of corrosivereducing agents;

(e) effecting reduction of the vanadium;

(f) recovering the resulting partially reduced catalyst precursor fromthe organic liquid medium;

(g) drying the partially reduced catalyst precursor;

(h) calcining the partially reduced catalyst precursor.

The catalysts prepared by the above process are particularly effectivein the oxidation of 4-carbon atom hydrocarbons such as n-butane,n-butenes, 1,3-butadiene or mixtures thereof with molecular oxygen or anoxygen-containing gas in the vapor phase to produce high yields ofmaleic anhydride with high selectivity. Essentially all the productproduced in this oxidation process is maleic anhydride, with only minoramounts of lower acids being detected.

DETAILED DESCRIPTION OF THE INVENTION

In the present process for the preparation of an oxidation catalystcontaining the mixed oxides of vanadium and phosphorus, a pentavalentvanadium compound and a phosphorus compound, also preferablypentavalent, are introduced into an aqueous medium.

Suitable vanadium compounds containing pentavalent vanadium includevanadium pentoxide and ammonium metavanadate. Suitable pentavalentphosphorus compounds include: phosphoric acid, phosphorus pentoxide or amixed pentavalent phosphorus component comprising a mixture oforthophosphoric acid and pyrophosphoric acid. Optionally, minor amountsof higher polyphosphoric acid may be included. If a mixed phosphorussource is desired, the phosphorus component mixture should compriseabout 45 to about 90 percent orthophosphoric acid, about 10 to about 50percent pyrophosphoric acid, and 0 to about 10 percent triphosphoricacid and higher polyphosphoric acids, percentages being based uponweight of total phosphoric acids. As hydrolysis is a factor indetermining the ratio of orthophosphoric acid to pyrophosphoric acidwhen present in aqueous solution, the above weight ratios aresignificant provided an extended period of hydrolysis has not occurredto convert the pyrophosphoric acid and higher polyphosphoric acids tothe orthophosphoric form.

The aqueous medium liquid may consist essentially of water, such thatthe medium comprises an aqueous solution or slurry of catalyst componentelement containing compounds including compounds containing vanadium,phosphorus, and promoter metal elements, if any. The aqueous mediumshould be free from agents which would substantially reduce thepentavalent vanadium component.

The manner of introduction of the pentavalent vanadium and phosphoruscompounds into the aqueous medium may vary. For example, both may beintroduced into water or an aqueous solution or slurry together, thephosphorus component may be introduced into an aqueous solution orslurry of the vanadium compound, or the vanadium compound may beintroduced into an aqueous solution of a phosphorus compound.

The substantially pentavalent, alpha-VOPO₄ catalyst precursor may beformed in the aqueous medium by heating the medium, preferably underreflux conditions. The pentavalent catalyst precursor may be recoveredby conventional methods, such as evaporation, precipitation andfiltration, centrifugation, and the like.

After recovery, the pentavalent alpha-VOPO₄ catalyst precursor isintroduced into the organic liquid medium for reduction of the vanadium.The organic liquid medium employed in the process of the presentinvention must be capable of reducing at least a portion of the vanadiumto a +4 valence state, preferably upon mixing and heating. The organicliquid medium should not, however, be a solvent for the mixed oxideprecursor of vanadium and phosphorus and thus the medium is maintainedfree of corrosive reducing or solubilizing agents such as HCl, HBr andoxalic acid.

Suitable organic liquid media for use in the invention include alcohols,aldehydes, ethers, glycols, ketones, halogenated olefins, mixturesthereof, and are preferably anhydrous. Suitable alcohols may be primaryor secondary, saturated or unsaturated. Examples of organic liquidssuitable for use in this invention include but are not limited toisopropanol, isobutanol, sec-butanol, allyl alcohol, crotyl alcohol,acetaldehyde, methyl ethyl ketone, ethylene glycol, dibutyl ether,hexachlorobutadiene, perchloropropene, and the like.

The reduction of the vanadium is preferably effected by heating theprecursor containing organic liquid medium, with stirring if desired.Preferred vanadium and phosphorus oxide catalysts for the oxidation of4-carbon atom hydrocarbons to maleic anhydride contain vanadium in anaverage valence state of about +3.5 to about +4.6. This average valencestate is achieved when at least a portion of the pentavalent vanadiumpresent in the organic liquid medium is reduced to the +4 state. Theaverage valence state of the vanadium is reduced preferably to about+4.1. In one embodiment of this invention, during the reduction of thevanadium in the organic liquid medium with heating under refluxconditions, at least 1.5 moles organic liquid per mole of vanadiumreacted is removed by distillation during the reaction of the vanadium,as taught by U.S. Ser. No. 286,434, assigned to our common assigneeherein.

During reduction of the vanadium in the alpha-VOPO₄ containing organicliquid medium, the partially reduced vanadium phosphorus mixed oxidecatalyst precursor is formed. After the partially reduced catalystprecursor is formed, it is recovered from the reaction medium byconventional methods including evaporation, filtration, centrifugationand decantation. The partially reduced catalyst is dried, generally attemperatures between about 100° C. to about 175° C. and is thereaftercalcined.

It is within the scope of this invention, to include promoterelement-containing compounds in the catalyst at a suitable point, suchas either by introducing the promoter element containing compounds inthe aqueous medium or in the organic liquid medium, prior or subsequentto the reduction of the vanadium, in order that the partially reducedcatalyst precursor contain the promoter element. Additionally, promoterelements may be added to the catalyst precursor after drying or to thecatalyst after calcination. Suitable promoters include but are notlimited to Ti, Cr, W, Nb, Ta, Mn, Th, U, Co, Mo, Fe, Zn, Hf, Zr, Ni, Cu,As, Sb, Te, Bi, Sn, Ge, Cd, the lanthanides or mixtures thereof.Suitable promoter element containing compounds include metal oxides,hydroxides, and salts such as nitrates, carbonates, acetates and thelike.

Catalysts produced by the process of this invention are preparedpreferably to exhibit a phosphorus to vanadium ratio of about 0.8:1 toabout 1.3:1. The molar ratio of promoter to vanadium is generallybetween about 0.01 to about 0.5. The catalyst is generally calcined inan inert atmosphere, air or an oxygen-containing gas at a temperature ofabout 250° C. to about 600° C. Calcination of the catalyst may also beaccomplished by heating the catalyst in a mixture of steam and air orair alone at a temperature of about 300° C. to about 500° C. Thecatalyst may also be calcined either in the presence of hydrocarbon, aninert gas, or both.

The hydrocarbon reacted to form maleic anhydride may be n-butane, then-butenes, 1,3-butadiene or a mixture thereof. Preferred is the use ofn-butane or a mixture of hydrocarbons that are produced in refinerystreams. The molecular oxygen needed for the reaction to produce maleicanhydride is most conveniently added as air, but synthetic streamscontaining molecular oxygen are also suitable. In addition to thehydrocarbon and molecular oxygen, other gases may be added to thereactant feed. For example, steam or nitrogen could be added to thereactants.

The ratio of the reactants may vary widely. The ratio of molecularoxygen to the hydrocarbon may range from about 3 to about 30 moles ofoxygen per mole of hydrocarbon. Preferred oxygen/hydrocarbon ratios areabout 4 to about 20 moles of oxygen per mole of hydrocarbon.

The reaction temperature may vary widely and is dependent upon theparticular hydrocarbon and catalyst employed. Normally, temperatures ofabout 250° C. to about 600° C. are employed with temperatures of about325° C. to about 475° C. being preferred.

The catalyst may be used alone or a catalyst support could be employed.Suitable supports include silica, alumina, silica-alumina, Alundum,silicon carbide, titania, boron phosphate, zirconia, and the like. Thecatalyst may be used in a fixed-bed reactor using tablets, pellets orthe like, or in a fluid-bed reactor using catalysts prepared such as byoil dropping or by spray drying, and preferably having a particle sizeof less than about 300 microns. The contact time may be as low as afraction of a second or as high as 50 seconds. The reaction may beconducted at atmospheric, superatmospheric or subatmospheric pressure.

SPECIFIC EMBODIMENTS OF THE INVENTION EXAMPLES 1-3

Catalysts of the formula V₁ P₁.16 O_(x) (where x is the number ofoxygens needed to satisfy the valence requirements of the otherelements) were prepared as follows. 90.95 g V₂ O₅ were added to 1.34 gH₃ PO₄ (85% aqueous) with stirring and slight heating. The resultingslurry of alpha-VOPO₄ was heated to evaporation and dryness at about150° C. for 16 hours. The dried alpha-VOPO₄ precursor was crushed andintroduced into about 700 ml isobutanol. The resulting slurry was heatedunder reflux conditions for about 16 hours.

The partially reduced catalyst precursor, a light blue fine precipitate,was filtered from the organic liquid medium, and about 55 g was dried at100° C. for about 1 hour and then at 150° C. for about 1 hour. The driedprecursor was calcined in air at 400° C. for about 1.5 hours. About 40 gof the calcined catalyst was mixed with about 1.2 g stearic acidprocessing aid and was tabletted to form 3/16 inch diameter (0.48 cm)tablets. The catalysts were tested for butane oxidation to maleicanhydride according to the procedure set forth below.

EXAMPLES 4-6

Catalysts of the formula V₁ P₁.16 O_(x) were prepared as follows. 90.95g V₂ O₅ and 133.8 g H₃ PO₄ (85% aqueous) were added to 1200 ml waterwith stirring. The mixture was heated to reflux for about 16 hours. Anadditional 268 g H₃ PO₄ (85%) was added with refluxing to precipitateout the alpha-VOPO₄ catalyst precursor. The resulting alpha-VOPO₄precipitate was recovered by filtration, and was heated to dryness at150° C.

About 110 g of the alpha-VOPO₄ precursor was introduced into 1200 mlisobutanol, and heated to reflux to effect vanadium reduction. Theresulting partially reduced blue catalyst precursor precipitate wasrecovered by filtration and heated to dryness at about 150° C. 50 g ofthe partially reduced catalyst precursor was calcined in air at about400° C. for one hour. About 42.5 g of the calcined catalyst was mixedwith a tabletting processing aid and was tabletted to form 3/16 inchdiameter (0.48 cm) tablets. The catalysts were tested for butaneoxidation to maleic anhydride according to the procedure set forthbelow.

Comparative Examples 7 & 8

Catalysts of the formula V₁.0 P₁.15 O_(x) were prepared according to theprior art method described above, in which the catalyst precursor isformed by reducing vanadium in a corrosive reducing agent-containingaqueous medium. 33.6 g of vanadium pentoxide were digested in 437.5 mls.of concentrated hydrochloric acid and refluxed for 3 to 4 hours. To thismixture was added 48.65 g of 85% phosphoric acid and refluxing wascontinued an additional 6 hours. The resulting mixture was evaporated todryness, and dried overnight at 110° C. Calcination was conducted for 1hour at 360° C. in air. The resulting catalysts were tested for butaneoxidation to maleic anhydride according to the procedure set forthbelow. These catalysts were much less active as compared to catalystsprepared by the method of the present invention, requiring higheroperating temperatures and higher air/hydrocarbon ratios, yet providinglower yields of product.

Comparative Examples 9 & 10

Catalysts of the formula V₁ P₁.16 O_(x) were prepared by heating amixture of 90.95 g V₂ O₅ and 133.74 g H₃ PO₄ (85% aqueous). To thepentavalent precursor was added the reducing agent 13 g N₂ H₄.H₂ O in 20ml water with heating. The vanadium was reduced to a valence state ofabout +4, and the resulting blue precipitate was filtered, washed withwater and dried for 2 hours at 150° C. The catalyst precursor wascalcined at 400° C. for about 75 minutes, and tested for butaneoxidation to maleic anhydride by the procedure set forth below. Thecatalysts prepared by sequentially forming a pentavalent VOPO₄ precursorand reducing the precursor with an inorganic reducing agent resulted ina catalyst having poor oxidation properties for production of maleicanhydride from n-butane.

Catalyst Testing

The catalysts described in Examples 1-10 were tested for the productionof maleic anhydride from n-butane using a 20 cc fixed-bed reactorconsisting of a 38 cm length of stainless steel tubing having an outerdiameter of about 1.3 cm and having a full length 0.31 cm axialthermowell. The reactor was heated with a split stainless steel blockfurnace. Flasks for receiving the product maleic anhydride were mountedin ice water, and tail gases were routed to a gas chromatograph foranalysis. Reaction conditions and results of the tests run are describedin Table I. The results are stated in terms as ##EQU1##

                  TABLE I                                                         ______________________________________                                        OXIDATION OF N--BUTANE                                                        TO MALEIC ANHYDRIDE OVER                                                      VANADIUM PHOSPHORUS MIXED OXIDE CATALYSTS                                                            Maleic                                                                        Anhydride                                              Exam-          Temper-  Contact                                                                              %           %                                  ple   Air/HC   ature    Time   Con-  %     Selec-                             No.   Ratio    °C.                                                                             (Sec.) version                                                                             Yield tivity                             ______________________________________                                        1     53.5     403      2      96.9  58.9  60.8                               2     52       393      2      85.4  56.7  66.5                               3     66.6     414      2      85.0  54.8  64.5                               4     62.9     415      1      92.1  55.2  60.0                               5     61       390      2      93.8  60.1  64.1                               6     61.4     398      2      84.5  56.1  66.4                               C7    112      499      1.3    55.8  30.4  54.5                               C8    108      513      1.4    67.5  31.8  47.1                               C9    64       400      1      5     --    --                                  C10  64       500      1      35    8     22                                 ______________________________________                                    

The results reported in the Table demonstrate that vanadium andphosphorus mixed oxide containing catalysts prepared by sequentiallyforming an alpha-VOPO₄ catalyst precursor in aqueous media, andthereafter partially reducing the vanadium present in the precursor inan organic liquid medium, in the absence of corrosive reducing agents,results in a catalyst exhibiting excellent activity for the oxidation ofhydrocarbons, particularly, the production of maleic anhydride from4-carbon atom hydrocarbons such as n-butane.

Thus it should be apparent to those skilfled in the art that the subjectinvention accomplishes the objects set forth above. It is to beunderstood that the subject invention is not to be limited by theexamples set forth herein. These have been provided merely todemonstrate operability, and the selection of vanadium compounds,phosphorus compounds, reducing organic liquid media, promoterelement-containing compounds, if any, hydrocarbon feedstocks andreaction conditions can be determined from the total specificationdisclosure provided without departing from the spirit of the inventionherein disclosed and described, the scope of the invention includingequivalent embodiments, modifications and variations that fall withinthe scope of the attached claims.

We claim:
 1. A process for the preparation of vanadium phosphorus mixedoxide containing catalysts comprising(a) introducing a pentavalentvanadium compound and a pentavalent phosphorus compound into an aqueousmedium wherein the aqueous medium is free from agents which wouldsubstantially reduce the pentavalent vanadium compound; (b) forming asubstantially pentavalent, alpha-VOPO₄ catalyst precursor in the aqueousmedium; (c) recovering the pentavalent catalyst precursor from theaqueous medium; (d) introducing the pentavalent catalyst precursor intoa substantially organic liquid medium capable of reducing at least aportion of the vanadium to a valence state of about +4 in the absence ofcorrosive reducing agents; (e) effecting reduction of the vanadium; (f)recovering the resulting partially reduced catalyst precursor from theorganic liquid medium; (g) drying the partially reduced catalystprecursor; (h) calcining the partially reduced catalyst precursor.
 2. Aprocess as in claim 1 wherein said organic liquid is selected fromalcohols, ethers, glycols, ketones, halogenated olefins, and mixturesthereof.
 3. A process as in claim 2 wherein said organic liquid isselected from isopropanol, isobutanol, sec-butanol, allyl alcohol,crotyl alcohol, acetaldehyde, methyl ethyl ketone, ethylene glycol,dibutyl ether, hexachlorobutadiene and perchloropropene.
 4. A process asin claim 2 wherein said organic liquid comprises an alcohol.
 5. Aprocess as in claim 4 wherein said alcohol comprises isobutanol.
 6. Aprocess as in claim 1 wherein reduction is effected by heating.
 7. Aprocess as in claim 1, including introducing a promoter elementcontaining compound into the aqueous medium, or into the organic mediumprior or subsequent to the reduction of the vanadium.
 8. A process as inclaim 7, wherein said promoter element is selected from Ti, Cr, W, Nb,Ta, Mn, Tb, U, Co, Mo, Fe, Zn, Hf, Zr, Ni, Cu, As, Sb, Te, Bi, Sn, Ge,Cd, the lanthanides or mixtures thereof.
 9. An oxidation catalystcontaining the mixed oxides of vanadium and phosphorus prepared by(a)introducing a pentavalent vanadium compound and a pentavalent phosphoruscompound into an aqueous medium wherein the aqueous medium is free fromagents which would substantially reduce the pentavalent vanadiumcompound; (b) forming a substantially pentavalent, alpha-VOPO₄ catalystprecursor in the aqueous medium; (c) recovering the pentavalent catalystprecursor from the aqueous medium; (d) introducing the pentavalentcatalyst precursor into a substantially organic liquid medium capable ofreducing at least a portion of the vanadium to a valence state of about+4 in the absence of corrosive reducing agents; (e) effecting reductionof the vanadium; (f) recovering the resulting partially reduced catalystprecursor from the organic liquid medium; (g) drying the partiallyreduced catalyst precursor; (h) calcining the partially reduced catalystprecursor.
 10. An oxidation catalyst as in claim 9 additionallycontaining a promoter element selected from Ti, Cr, W, Nb, Ta, Mn, Th,U, Co, Mo, Fe, Zn, Hf, Zr Ni, Cu, As, Sb, Te, Bi, Sn, Cd, Ge, thelanthanides or mixtures thereof.